You can get rid ones which are considered 'romless' (those which work without roms, including several consoles and computers) by selecting Configure Options --> Miscellaneous Options --> Hide romless machine from available list. Beyond that, you can use a filter or a saved favorite list to only show roms you want. The ultimate way is to simply move/delete your unwanted roms or put only roms in your rompath you want to see. The internal UI has no arbitrary way to eliminate what it finds in your rompaths.